Medical cutting tool

ABSTRACT

[Problem] To provide a method for manufacturing a medical cutting tool and a medical cutting tool that will not rust and that is capable of stably demonstrating high strength. 
     [Solution] A working section ( 2 ) formed by a cemented carbide or ceramic is brazed to the tip of a shank part ( 1 ) formed by a round bar-shaped austenitic stainless steel, thereby constituting a cutting tool (A). The shank part ( 1 ) comprises a shank ( 1   a ), a neck ( 1   b ), and a joint ( 1   c ) to which the working section ( 2 ) is brazed. At least part of the stainless steel near the joint ( 1   c ) of the neck ( 11   b ) and in the joint ( 1   c ) has a fiber-like structure. The structure of the stainless steel in the joint ( 1   c ) on the working section ( 2 ) side has a granular structure.

TECHNICAL FIELD

The present invention relates to a medical cutting tool which is used tocut a surface layer of a tooth, a bone including an alveolar bone, andthe like used during dental treatment.

BACKGROUND

In a dental treatment, a desired treatment may be performed by cutting asurface layer of a tooth. A medical cutting tool used during thetreatment includes a shank portion gripped by a chuck of a handpiece,and a working portion continuously formed from the shank portion andincluding a cutting blade which is formed from an outer circumference toa leading end portion and is used to cut a surface layer. The medicalcutting tool formed as described above is fixed to the handpiece whenthe shank portion is gripped by the chuck. Then, by an operation of adoctor, the rotating working portion is pressed to a portion to betreated, thereby cutting a desired portion.

In the medical cutting tool, the working portion needs to ensure asufficient cutting performance with respect to a hard surface layer.Thus, in general, the working portion is formed using cemented carbiderepresented by tungsten carbide. The shank portion needs to ensure astrength that can sufficiently resist bending and warping affected whena desired portion is cut by the working portion. When the workingportion is formed using cemented carbide to improve the strength of theshank portion, it is preferable that the shank portion be formedcontinuously from the working portion and using cemented carbide.

However, there occurs a problem that the time used to process thecemented carbide in a shape corresponding to the medical cutting tooltakes long. Thus, the shank portion is formed using, for example, SUS420 which is martensitic stainless steel or carbon tool steels (SK), anintermediate component used to form the working portion is formed bycemented carbide, and the intermediate component of the working portionis bonded to a leading end of the shank portion by pressure welding, sothat a medical cutting tool processed in a desired shape is provided.

As described above, when stainless steel or tool steel and cementedcarbide are bonded to each other, an optimum method corresponding to amaterial to be bonded is selected and employed from methods such aspressure welding including friction pressure welding and heatingpressure welding, resistance welding, and brazing in general.

SUMMARY OF INVENTION Technical Problem

When stainless steel or carbon steel forming the shank portion andcemented carbide forming the working portion are bonded to each otherusing friction pressure welding, resistance welding, and brazing, abonded portion is heated. Accordingly, a strength of a material formingthe shank portion may vary due to a thermal effect, and cemented carbidemay be oxidized. That is, an annealing effect may occur due to anincrease in temperature of the bonded portion, and thus the strength ofthe shank portion may decrease and an oxide film may be generated on asurface of cemented carbide.

When a material forming the shank portion is martensitic stainless steelor carbon steel that may expect hardening from a heat treatment, thestrength can be improved by performing heat treatment again after thebonding. However, martensitic stainless steel or carbon steel hasproblems that the generation of rust cannot be excluded and a fractureis likely to be generated when hardness is excessively increased by heattreatment.

In particular, from the viewpoint of preventing the generation of rust,it is preferable that austenitic stainless steel be used as a materialof the shank portion. However, this material may not have an improvedstrength from heat treatment. Thus, the strength is improved byextending a structure to a fiber shape by a cold drawing process.However, when a structure extended in a fiber shape changes to agranular structure, the strength may be degraded.

An object of the invention is to provide a medical cutting tool in whichrust is not be generated, a fracture is hardly generated, and a highstrength is stably exhibited.

Solution to Problem

A medical cutting tool according to the invention to resolve the aboveproblems is a medical cutting tool formed by brazing a working portionformed of cemented carbide or ceramic to a leading end of a shankportion formed by a round austenitic stainless steel bar, wherein theshank portion includes a shank, a neck formed to be continuous to theshank, and a joint which is formed to be continuous to neck and brazesthe working portion formed of cemented carbide or ceramic, and at leasta portion of stainless steel in the joint and near the joint of the neckincludes a structure of a fiber shape.

In the medical cutting tool described above, it is preferable that astructure of the stainless steel in the joint at a side of the workingportion be a granular structure.

Advantageous Effects of Invention

In the medical cutting tool according to the invention, the shankportion is formed of austenitic stainless steel, and includes the shank,the neck, and the joint, and at least a portion of stainless steel inthe joint and near the joint of the neck has a structure of a fibershape. Thus, it is possible to sufficiently resist bending or warpingacting when the working portion presses and cuts a hard layer.

In addition, since the structure at the side of the working portion ofstainless steel in the joint is a granular structure, the joint at theside of the working portion is in an annealing state, and thusflexibility can be exhibited and a fracture can be difficult to begenerated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a cutting tool.

FIG. 2 is an enlarged view illustrating a state in which a material of aworking portion is brazed to a brazing surface of a shank portion.

FIG. 3 is a diagram illustrating a configuration in which gas is sprayedto a brazing region and near the brazing region to perform cooling.

FIG. 4 is a diagram illustrating a process order when the cutting toolis manufactured.

FIG. 5 is a diagram illustrating a measurement position of hardness in aneck of the cutting tool.

FIG. 6 is a diagram illustrating a structure of the brazing region ofthe cutting tool.

REFERENCE SIGNS LIST

-   -   A Cutting tool    -   1 Shank portion    -   1 a Shank    -   1 b Neck    -   1 c Joint    -   1 d Brazing surface    -   2 Working portion    -   2 a Cutting blade    -   2 b Brazing surface    -   3 Brazing portion    -   5 a Apex    -   5 b Sloped surface    -   6 Solder    -   7 Material    -   8 Intermediate material    -   11 Chuck    -   12 Axis line    -   13 Holding member    -   13 a Holding hole    -   14 Heating member    -   15 Nozzle

DESCRIPTION OF EMBODIMENTS

Hereinafter, a medical cutting tool (hereinafter, referred to as a“cutting tool”) of the invention will be described. The cutting tool ofthe invention is fixed to a chuck of a handpiece gripped and operated bya hand of a doctor to be rotated, so that a working portion presses asurface layer of a tooth or a surface of a bone including an alveolarbone to cut the surface layer or the surface.

In particular, a favorable cutting performance is ensured by forming theworking portion using cemented carbide or ceramic (hereinafter,represented by and referred to as “cemented carbide”), and at least aportion of stainless steel in a joint and near the joint of a neckforming a shank portion which is constructed by a round austeniticstainless steel bar (hereinafter, simply referred to as “stainlesssteel”) is formed in a structure of a fiber shape, thereby exhibitinghigh durability.

In addition, in the cutting tool of the invention, a structure on a sideof the working portion of a joint forming the shank portion is formed ina granular structure, thereby causing a side of the brazing portion inthe joint to exhibit flexibility so that a fracture is hardly generated.

In the cutting tool according to the invention, the shank portion isformed using a round stainless steel bar. The shank portion includes ashank, a neck formed to be continuous to the shank, and a joint formedto be continuous to the neck. In the shank portion, at a stage of anintermediate component preceding brazing of a material of cementedcarbide forming the working portion, the shank and the neck are providedin the substantially same shape and size as those of a desired cuttingtool, and the joint is provided in a size corresponding to a size of amaterial to be brazed. The shank portion is extremely thin, where theshank which is the thickest portion is about 1.6 mm and the thinnestportion in the neck is about 0.4 mm.

The neck forming the shank portion has a function of smoothly connectingfrom the thickness of the shank to the thickness of the working portion(thickness of the joint). In a general cutting tool, the thickness ofthe shank is greater than the thickness of the working portion, and thusthe neck is formed in a tapered shape where a diameter decreases fromthe shank to the joint.

Since a great stress acts on the neck while a desired treatment isperformed, a high strength is needed to be exhibited. The strength inthe neck can be identified by conducting a test such as a tensile testand a flexural test. However, the inventor verifies a relation among theVickers hardness, the strength needed for the cutting tool, and theextension of a fiber of a structure entailed by a cold drawing processas a result of various experiments conducted in a developmental processof the cutting tool over the years.

That is, after the cold drawing process, it is verified that both agranular structure and a fiber shaped structure are included when theVickers hardness of stainless steel is around 200, and a structure isextended in a fiber shape when the Vickers hardness is greater than orequal to 250. In addition, it is verified that it is possible tosufficiently resist bending and a warping that act on the neck when theVickers hardness of stainless steel in the neck is greater than or equalto 350 and the working portion presses and cuts a tooth. However, it isverified that the Vickers hardness has an upper limit which is about 650at the maximum, and in particular, it is preferable that the Vickershardness is in the range of 350 or more and 600 or less because thereoccurs a problem that a fracture is easily generated when the hardnessis excessively high.

Accordingly, stainless steel forming the shank portion needs to be ableto have the Vickers hardness in the neck in the range of 350 or more and650 or less. For this reason, the cold drawing process is performed inadvance on the stainless steel forming the shank portion, and astructure is extended in a fiber shape. In this way, by extending thestructure in a fiber shape, it is possible to form at least a portion ofstainless steel in the joint and near the joint of the neck in astructure of a fiber shape.

Further, the surface of the stainless steel, in which a structure isextended in a fiber shape by performing the cold drawing process, needsto have a sufficiently great value than the Vickers hardness of 350.When the surface of austenitic stainless steel has such hardness, it ispossible to provide the Vickers hardness of the neck of 350 or more andthe fiber shaped structure by cutting a portion corresponding to theneck in forming the shank portion.

Tungsten carbide (WC) or titanium carbide used as a cutting tool inmachine processing may be employed as cemented carbide forming theworking portion, and carbide ceramics represented by cermet may beemployed as ceramic. The cemented carbide is brazed to the joint formedat a leading end of the shank portion in a state of a material.Thereafter, in a state in which the shank is gripped, machine processingis performed to have a size corresponding to a desired cutting tool, andmachine processing of forming a spiral-shaped cutting blade isperformed. Therefore, when a material of the working portion is brazedto the shank portion, the shank and the neck forming the shank portionsubstantially have a shape and a size corresponding to a desired cuttingtool. However, a material of the joint and the working portion does nothave a shape of the desired cutting tool, and has a large size.

In the invention, a material of a solder used when the joint of theshank portion and a material of the working portion are brazed is notparticularly limited. However, considering an influence of a brazingtemperature with respect to the shank portion, a silver solder thatmelts at a relatively low temperature (for example, about 700° C.) ispreferable. In addition, a property of the solder is not particularlylimited, and any of a foil shape, a bar shape, and a paste shape may beused. However, a solder in a paste shape is preferable since a thicknessof the joint of the shank portion or a material of the working portionis thin, and a void is small.

When a material of the working portion is brazed to the joint of theshank portion, typically, cemented carbide which is the material of theworking portion is heated in general. In this case, examples of a schemeof heating the material of the working portion formed of cementedcarbide or ceramic include a heat scheme using a flame, a heat scheme ofaccommodating the material in a holding furnace, a heat scheme using anelectromagnetic induction, and the like, and a scheme may be selectedand employed from the schemes.

In the invention, the material of the working portion is brazed to thejoint forming the shank portion. Thus, due to heat given during thebrazing, a temperature of the joint on the side of the working portionincreases to an annealing temperature, and the temperature of the jointand the neck near the joint increases to an annealing temperaturedepending on a heating temperature and a heating time.

Since the structure of stainless steel forming the shank portion isextended in a fiber shape, the joint on the side of the working portionis annealed so that the structure becomes a granular structure. When thestructure of the joint on the side of the working portion is a granularstructure, the granular structure portion exhibits flexibility so that afracture is hardly generated.

In particular, since the structure is stainless steel extended in afiber shape, due to heat entailed by a brazing of cemented carbide whichis a material of the working portion for the joint, both the joint andthe neck are annealed, and thus a structure of a fiber shape may becomea granular structure. Thus, it is preferable that a brazing region ofstainless steel forming the shank portion and the neighborhood beforcibly cooled down.

As described above, by forcibly cooling down the brazing region ofstainless steel including the structure extended in a fiber shape whileperforming brazing, it is possible to prevent the temperature of theshank portion including the joint from excessively increasing.Accordingly, the structure of the fiber shape in the neck can bemaintained, and the strength of the neck can be maintained.

Next, an overall configuration of the cutting tool according to theembodiment will be described with reference to FIG. 1.

Referring to FIG. 1, a cutting tool A includes a shank portion 1 fixedto a chuck of a handpiece (not illustrated), and a working portion 2that cuts a surface layer of a tooth (not illustrated). The shankportion 1 is made from stainless steel in which a structure is extendedin a fiber shape, and the working portion 2 is made from WC as amaterial. The cutting tool A is constructed when the shank portion 1 andthe working portion 2 are connected and integrated with each otherthrough a brazing portion 3 using brazing.

The shank portion 1 of the cutting tool A is fixed to the chuck of thehandpiece (not illustrated), and rotates in this state. For this reason,a shank 1 a fixed to the chuck of the handpiece is formed on one side ofthe shank portion 1. The shank 1 a has a constant thickness (a thicknessof about 1.6 mm in the embodiment) corresponding to the size of thechuck regardless of the shape, the thickness, and the length of theworking portion 2.

A specification such as an external shape, a thickness, and a length ofthe working portion 2 in the cutting tool A is set according to atherapeutic purpose of a portion or a degree to be cut. Thus, thecutting tool A illustrated in FIG. 1 is merely an example of the cuttingtool A. For example, a shape of the working portion 2 includes a roundbar shape illustrated in FIG. 1 in which a leading end has a sphericalshape and a thickness is uniform, a tapered shape in which a leading endhas a spherical shape and a thickness becomes thinner toward the leadingend, a sphere shape, and the like. A plurality of spiral-shaped cuttingblades 2 a is formed on an outer circumference surface of the workingportion 2.

As described above, an external diameter of the working portion 2 is setto various values corresponding to the purpose of the cutting tool A.Thus, as illustrated in FIGS. 2 and 4, a material 7 of cemented carbideused to form the working portion 2 corresponding to the cutting tool A(hereinafter, simply referred to as “material 7”) is brazed to theleading end of the shank portion 1 corresponding to the desired cuttingtool A in advance, and then machine processing such as cutting andgrinding is performed on the leading end portion of the shank portion 1and the material 7, thereby providing a shape and a size correspondingto the desired working portion 2.

Thus, in the shank portion 1, a neck 1 b in a tapered shape is formed tobe continuous to the shank 1 a having a constant thickness, and a joint1 c having a diameter substantially equal to or slightly less than athickness of the material 7 is formed at a leading end of the neck 1 b.A thickness of the joint 1 c is not particularly limited, and one thirdof a diameter of the joint 1 c or less is sufficient for the thickness.As illustrated in FIG. 2, a brazing surface 1 d is formed on an edgeface of the joint 1 c, and a sloped surface 5 b having an apex 5 a isformed on the brazing surface 1 d.

The sloped surface 5 b having the apex 5 a formed on the brazing surface1 d of the shank portion 1 is formed as a sloped surface having acircular cone shape for an easiness of processing. By forming the slopedsurface 5 b in a circular cone shape, the brazing surface 1 d may befabricated concurrently with a fabrication of another portion, forexample, the neck 1 b and the joint 1 c when the shank portion 1 isrotated and cut from a stainless steel material having a round barshape, which is advantageous.

As a result of various experiments conducted by the inventor, as one ofconditions for being able to exhibit a practical bending strength by thebrazing portion 3, a relation between a height of the sloped surface 5 bin the brazing portion 3 and a thickness of the brazing portion 3 isconfirmed to be present. That is, referring to the relation, it ispreferable that the height of the sloped surface 5 b be set in the rangeof 0.5% to 8% of the diameter of the brazing surface. When the height isless than a value in the range, an amount of solder is small, and thusthe sufficient joint strength is difficult to be obtained. In addition,when the height is greater than a value in the range, an amount ofsolder is large, and thus workability deteriorates, cost increases, andvariation occurs in a joint strength. In particular, to obtain a uniformand sufficiently great joint strength, it is preferable that the heightof the sloped surface 5 b provided on the brazing surface 1 d be set inthe range of 1% to 5% of a diameter of the brazing surface 1 d.

For this reason, in a case of the cutting tool A in which the thicknessof the working portion 2 is 1.2 mm, the height of the sloped surface 5 bis 0.038 mm, and is set to be about 3.1% of the thickness of the workingportion 2. In addition, in a case of the cutting tool A in which thethickness of the working portion 2 is 1.0 mm, the height is 0.031 mm,and is set to be about 3.1% of the thickness of the working portion 2.Further, in a case of the cutting tool A in which the thickness of theworking portion 2 is 0.8 mm, the height is 0.022 mm, and is set to beabout 2.8% of the thickness of the working portion 2.

In addition, the brazing surface 2 b is formed on an edge face at theside of the shank portion 1 of the working portion 2 (material 7). Theshape of the brazing surface 2 b is not particularly limited, and may bea flat surface or the same shape as that of the brazing surface 1 d ofthe shank portion 1. In the embodiment, as illustrated in FIG. 2, thebrazing surface 2 b of the working portion 2 is formed as a flatsurface.

The shank portion 1 and the material 7 are disposed in which the brazingsurface 1 d formed at the joint 1 c of the shank portion 1 and thebrazing surface 2 b formed on an edge face of the material 7 face eachother in the brazing portion 3, and are bonded to each other by thesolder 6 supplied to a void formed between the brazing surfaces 1 d and2 b and hardened.

In particular, by causing the apex 5 a of the sloped surface 5 b formedon the brazing surface 1 d of the shank portion 1 to come into contactwith the brazing surface 2 b of the material 7, the void between thebrazing surfaces 1 d and 2 b is set to a size identical to the height ofthe sloped surface 5 b. Accordingly, a volume of the void in the brazingportion 3 is substantially constant, and a supplied amount of the solder6 may be stable.

When the shank portion 1 and the material 7 are brazed and bonded toeach other, the brazing portion 3 is heated to a melting temperature ofthe solder 6, and the solder 6 is melted due to an increased temperaturein the brazing portion 3, so that the void formed between the brazingsurfaces 1 d and 2 b is filled. Then, when heating of the brazingportion 3 is suspended, the brazing portion 3 is cooled down, and thesolder 6 is hardened, so that the shank portion 1 and the material 7 arebonded to each other. Thereafter, the cutting tool A is constructed byperforming predetermined machine processing on the material 7.

In the cutting tool A constructed as described above, the neck 1 b hasthe hardness greater than or equal to the Vickers hardness 350, and itis possible to exhibit the sufficient strength with respect to thebending stress and the torsional force acting when a desired cuttingoperation is performed by the working portion 2.

For example, when the Vickers hardness of the neck 1 b is less than orequal to 350, the neck portion is bent, and a stable cutting isdifficult, which is not preferable. However, an unlimitedly highhardness of the neck 1 b is not preferable. For example, when theVickers hardness exceeds 650, a fracture is easily generated, which isnot preferable.

Next, a procedure of manufacturing the cutting tool A constructed asdescribed above will be described. In the embodiment, while a brazingoperation is performed, a neighborhood of the brazing region is forciblycooled down, thereby preventing an annealing of the neck 1 b in theshank portion 1 to maintain a structure extended in a fiber shape, andmaintaining the Vickers hardness greater than or equal to 350.

In the embodiment, the joint 1 c and a joint part between the joint 1 cand the neck 1 b are collectively referred to as a brazing region. Inaddition, a neighborhood of the joint 1 c in the neck 1 b includes thejoint part between the joint 1 c and the neck 1 b, is a portion includedin a neighborhood of the brazing region, and is a portion about twice athickness of the joint 1 c.

When the material 7 of the working portion 2 is brazed to the shankportion 1 formed from stainless steel, a scheme of cooling down thebrazing region and the neighborhood of the brazing region are notparticularly limited. A cooling scheme that may maintain the brazingregion and the neighborhood of the brazing region at a temperature lessthan or equal to a transformation temperature of a structure may beused. That is, a transformation of austenitic stainless steel may occurfrom about 500° C., and thus a cooling scheme that may maintain thetemperature or less by forcibly cooling down the brazing region and theneighborhood of the brazing region in the shank portion 1 may beemployed.

Examples of the cooling scheme include a cooling scheme that sprays gashaving a pressure, a cooling scheme that sprays liquid as a mist, acooling scheme that directly sprays liquid, a cooling scheme that spraysa dry ice chip or an ice chip, and a cooling scheme that disposes acooling medium at the brazing region and the neighborhood of the brazingregion, and sprays wind through the cooling medium. A cooling scheme maybe selected and employed from the cooling schemes.

In addition, when the shank portion 1 is formed using stainless steel,and the working portion 2 is formed using cemented carbide which isformed from WC, it is preferable that an oxidization of a surface of thestainless steel and a surface of the cemented carbide occurring during abrazing operation be reduced as possible. For this reason, it ispreferable that an inert gas be sprayed when forcibly cooling down thebrazing region or the neighborhood of the brazing region in stainlesssteel.

In this case, it is preferable that gas generally used as an inert gassuch as nitrogen gas and argon gas be used as the inert gas. However, anobject of spraying an inert gas is to prevent an oxidization of asurface of stainless steel and a surface of cemented carbide, and thusgas which is inert at a degree capable of achieving the object may beused. That is, 100% nitrogen gas or argon gas may not be used as aninert gas, and gas in which nitrogen gas or argon gas and air are mixedmay be used.

In addition, when the shank portion 1 and the material 7 of the workingportion 2 are brazed, gas is generated due to an increased temperatureof a solder. Thus, as an example of the embodiment, by forming thesloped surface 5 b including the apex 5 a on the brazing surface 1 dformed on the edge face of the joint 1 c of the shank portion 1, thegenerated gas may be smoothly removed from a brazing part. Inparticular, when a brazing operation is performed, by causing the apex 5a to come into contact with the edge face of the material 7, a gaptherebetween may be maintained.

Further, when the shank portion 1 and the material 7 of the workingportion 2 are brazed, it is preferable that cemented carbide forming thematerial 7 be disposed in the bottom for supporting, and the shankportion 1 be substantially perpendicularly disposed to cause the apex 5a to come into contact with the edge face of the material 7. Whendisposed in this position, gas generated during a brazing rises alongthe sloped surface 5 b and is vented to atmosphere from an outercircumference of the joint 1 c. In addition, a melted solder penetratesinto a small part by capillary phenomenon acting on a minute void formedfrom a slope of the protrusion portion and the edge face of the workingportion, and an excellent brazing may be realized.

By providing the sloped surface 5 b including the apex 5 a on thebrazing surface 1 d of the shank portion 1, a gap between the brazingsurface 1 d and the edge face of the material 7 (brazing surface) may beset during a brazing operation, and the gap may be maintained throughthe brazing operation. Thus, the brazing operation may be performedwhile the gap between the brazing surface 1 d and the edge face of thematerial 7 is stably maintained, and a variation in strength resultingfrom the brazing operation may be excluded.

A shape of the sloped surface 5 b including the apex 5 a provided on thebrazing surface 1 d is not particularly limited. Examples of the shapeinclude a curved surface shape, a circular cone shape, a pyramid shape,and the like, and a shape including the shapes may be used. However,considering at least a condition of easiness and the like of processingwhen the shank portion 1 is formed, it is preferable to be formed as acircular cone shape.

The sloped surface 5 b including the apex 5 a provided on the brazingsurface 1 d of the shank portion 1 is brazed in a state of coming intocontact with an opposing brazing surface (the edge face of the material7). Thus, a height of the sloped surface 5 b defines a gap between twoopposing brazing surfaces, and sets a volume of a solder in the brazingportion. For this reason, it is preferable that the height of the slopedsurface 5 b be changed in response to the thickness in the joint portionbetween the shank portion 1 and the working portion 2.

A configuration of a brazing device suitably used when the shank portion1 and the material 7 forming the working portion 2 are brazed asdescribed above, and a procedure of a brazing using the brazing devicewill be described with reference to FIG. 3. The brazing deviceillustrated in FIG. 3 is configured so as to be able to forcibly cooldown a brazing region of the shank portion 1 and the material 7, and aneighborhood of the brazing region by spraying an inert gas.

Referring to FIG. 3, a chuck 11 is configured to be able to reciprocatealong an axis line 12 by gripping the shank 1 a of the shank portion 1.A holding member 13 is configured to be able to put and hold thematerial 7 of the working portion 2 in a holding hole 13 a formed on theaxis line 12. A heating member 14 is configured to be able to heat thematerial 7 held in the holding hole 13 a of the holding member 13 by acoil in a ring shape.

A nozzle 15 is configured to spray an inert gas to forcibly cool down abrazing region of the brazing surface 1 d formed at the joint 1 c of theshank portion 1 and the material 7 of the working portion, and aneighborhood of the brazing region. As the inert gas sprayed from thenozzle 15, nitrogen gas, argon gas, and the like may be selectivelyused. For this reason, a source of supply of an inert gas (notillustrated) is connected to the nozzle 15.

The number of nozzles 15 is not limited, and may be one, or two or more.The nozzle 15 is needed to be disposed to be able to evenly cool down abrazing region in the shank portion 1 and the neck 1 b which is aneighborhood of the brazing region. In addition, a temperature of aninert gas sprayed from the nozzle 15 is not limited, and a sufficientcooling effect may be exhibited even when an inert gas in a normaltemperature state is sprayed.

An amount of inert gas sprayed from the nozzle 15 is preferably large.However, it is preferable to be appropriately set from a relation with ascheme of heating the brazing portion 3. For example, in a case of aheating scheme using a combustion flame of gas, a stable combustionflame may be difficult to be formed when a flow rate of an inert gasincreases, and a current speed increases. However, in a case of a schemeusing an induction heating as an example of the embodiment, a highcooling effect may be exhibited by spraying an inert gas of asufficiently large flow rate.

As illustrated in FIGS. 2 and 3, first, the material 7 is put into theholding hole 13 a of the holding member 13, and the shank 1 a of theshank portion 1 is gripped by the chuck 11. The solder 6 is supplied tothe brazing surface 2 b which is the edge face of the material 7, thechuck 11 is caused to descend along the axis line 12, and the apex 5 aof the sloped surface 5 b formed on the brazing surface 1 d of the shankportion 1 is caused to come into contact with the brazing surface 2 b.

When an inert gas is sprayed from the nozzle 15 by maintaining thestate, the brazing region in the shank portion 1 and the neck 1 b whichis a neighborhood of the brazing region are cooled down.

Further, the material 7 of the working portion 2 is heated for a shortperiod of time by operating the heating member 14, and the solder 6 ismelted due to the heating to flow to and fill the void formed by thesloped surface 5 b provided on the brazing surface 1 d of the shankportion 1, and the edge face of the material 7. In addition, gasgenerated due to the melting of the solder 6 moves upward along thesloped surface 5 b, arrives at the circumference surface of the joint 1c, and then is vented to atmosphere.

The heating member 14 is operated for a predetermined time (for example,about greater than or equal to 1 second and less than or equal to 5seconds), and the operation of the heating member 14 is suspended afterthe melted solder 6 sufficiently flows to fill the void formed by thesloped surface 5 b and the edge face of the material 7. The solder 6 ishardened due to the suspension of the operation of the heating member14, and a brazing operation is ended.

As described in the foregoing, while the heating member 14 is operated,and the brazing operation is performed, an inert gas is sprayed to thebrazing region in the shank portion 1 and the neck 1 b which is aneighborhood of the brazing region to forcibly cool down the brazingregion and the neck 1 b. For this reason, even though heat entailed bythe brazing is transferred to the neck 1 b, an increase in temperaturemay be inhibited, and recrystallization of a structure extended in afiber shape may be prevented. Since a structure of the neck 1 bmaintains a state of being extended in a fiber shape, the Vickershardness of 350 or more is maintained, and a strength as the cuttingtool A may be maintained.

As describe above, the material 7 of the working portion 2 brazed to theshank portion 1 may not have a shaft center accurately matching a shaftcenter of the shank portion 1. For this reason, as illustrated in FIG.4, the desired cutting tool A is manufactured by performing machineprocessing including a cutting or a grinding on the material 7.

That is, as illustrated in FIG. 4( a), the material 7 of cementedcarbide sufficiently greater than a thickness and a length of thedesired working portion 2 is brazed to the brazing surface 1 d formed atthe joint 1 c of the shank portion 1, and then an intermediate material8 is constructed by processing the material 7 as illustrated in FIG. 4(b). This operation is fixing the shank 1 a of the shank portion 1 to aprocessing equipment (not illustrated), and grinding the material 7 sothat the working portion 2 has a desired thickness while causing arotation in this state. As described above, since a center of rotationof the shank 1 a of the shank portion 1 is a center of rotation of thecutting tool A, a center of rotation of the intermediate material 8accurately matches the center of rotation of the shank portion 1 evenwhen a center of rotation of the material 7 does not match the center ofrotation of the shank portion 1.

Next, as illustrated in FIG. 4( c), the intermediate material 8 isprocesses to form a hemispherical edge, and the spiral-shaped cuttingblades 2 a are formed on a circumference surface. Through theprocessing, the working portion 2 is formed of the material 7 throughthe intermediate material 8, thereby manufacturing the desired cuttingtool A.

The inventors measured the Vickers hardness of the neck 1 b in thecutting tool A formed by forcibly cooling down the brazing region in theshank portion 1 and the neck 1 b which is a neighborhood of the brazingregion, and the Vickers hardness of the neck 1 b in a cutting toolformed without forcibly cooling down the neck 1 b, and compared theVickers hardness. The result will be described.

Ten cutting tools of a specification illustrated in FIG. 5 arefabricated as test pieces. A minimum size of the neck 1 b in FIG. 5 is1.02 mm. Five test pieces among the ten test pieces are fabricated byspraying nitrogen gas of 5 normal liters per minute to the brazingregion in the shank portion 1 and a neighborhood of the brazing region,and forcibly cooling down the brazing region and the neighborhood. Theremaining five pieces are fabricated without being cooled down.

As a brazing region in each test piece, an intersection part of the neck1 b and the joint 1 c of the shank portion 1 is set, and the hardness ismeasured for four points (0, 0.7, 1.4, and 2.1) from this part for each0.7 mm. A measurement scheme of hardness is based on JIS Z2244 (2003).The Vickers hardness is HV0.3, a test pressure is 2.942 N, and a holdingtime of the test pressure is 15 seconds.

As a result of the test, the Vickers hardness of the five cooled piecesis in a range of 367 to 412 at the point 0, in a range of 475 to 503 atthe point 0.7, in a range of 504 to 533 at the point 1.4, and in a rangeof 516 to 550 at the point 2.1. In addition, the Vickers hardness of thefive pieces that are not cooled is in a range of 285 to 299 at the point0, in a range of 334 to 343 at the point 0.7, in a range of 439 to 453at the point 1.4, and in a range of 484 to 508 at the point 2.1.

From a result of the test, it is clarified that the Vickers hardness of350 or more of the neck 1 b may be realized by forcibly cooling down thebrazing region and a neighborhood of the brazing region. In the neck 1 bhaving the Vickers hardness of 350 or more, it can be presumed that astructure maintains a state of being extended in a fiber shape.

In addition, ten cutting tools in which a length of the neck 1 b of FIG.5 is 6.3 mm, and a minimum size of the neck 1 b is 0.62 mm arefabricated as test pieces. Similarly to the above test, five test piecesamong the ten test pieces are fabricated by spraying nitrogen gas of 5normal liters per minute to the brazing region in the shank portion 1and a neighborhood of the brazing region, and forcibly cooling down thebrazing region and the neighborhood. The remaining five pieces arefabricated without being cooled down. Then, the hardness for four pointsare measured under the same condition as described above.

As a result of the test, the Vickers hardness of the five cooled piecesis in a range of 357 to 376 at the point 0, in a range of 460 to 478 atthe point 0.7, in a range of 533 to 544 at the point 1.4, and in a rangeof 550 to 558 at the point 2.1. In addition, the Vickers hardness of thefive pieces that are not cooled is in a range of 260 to 283 at the point0, in a range of 318 to 342 at the point 0.7, in a range of 404 to 430at the point 1.4, and in a range of 502 to 532 at the point 2.1.

From the result of the test, it is clarified that the Vickers hardnessof 350 or more of the neck 1 b may be realized by forcibly cooling downthe brazing region and a neighborhood of the brazing region even for thecutting tool in which a length of the neck 1 b is 6.3 mm, and a minimumsize of the neck 1 b is 0.62 mm. In the neck 1 b having the Vickershardness of 350 or more, it may be presumed that a structure maintains astate of being extended in a fiber shape.

In the cutting tool A formed as described above, the joint 1 c formingthe shank portion 1 at the side of the working portion is in a granularstructure. FIG. 6 is an enlarged view of the structure of the brazingportion 3 in a longitudinal direction (a direction that cuts the shankportion 1, the solder 6, and the working portion 2 longitudinally) ofthe cutting tool A. The joint 1 c at the side of the working portion 2is in a granular structure at a range of a depth of several μm, andanother portion maintains a structure of a fiber shape.

INDUSTRIAL APPLICABILITY

A cutting tool of the invention is formed by brazing and bonding aworking portion formed of cemented carbide or ceramics to a leading endof a shank portion formed by a round austenitic stainless steel bar, andat least a portion of stainless steel in a joint and near the joint ofthe neck includes a structure of a fiber shape, and thus the neck has asufficient strength for a bending and a warping. For this reason, it isadvantageous to be used when cutting a hard layer such as a tooth.

1. A medical cutting tool formed by brazing a working portion formed ofcemented carbide or ceramics to a leading end of a shank portion formedby a round austenitic stainless steel bar, wherein the shank portionincludes a shank, a neck formed to be continuous to the shank, and ajoint which is formed to be continuous to the neck and which brazes theworking portion formed of cemented carbide or ceramics, and at least aportion of stainless steel in the joint and near the joint of the neckincludes a structure of a fiber shape.
 2. The medical cutting toolaccording to claim 1, wherein a structure of the stainless steel in thejoint at a side of the working portion is a granular structure.